The invention relates to a valve and to a use of the valve. Valves, for example for gas ducts, are known. They often comprise a housing with a valve seat in which a rotatably mounted flap is arranged for closing the gas duct. In order to avoid leakage air, the flap is provided with a piston seal encircling it on the outside. The latter generally comprises a metal ring which, for flexibility reasons, has a gap. During operation, it is often disadvantageous that leakage air can pass through this gap, which is not desirable. This is the case in particular if the gases to be conducted by the valve have relatively high temperatures and in addition greatly fluctuate in their temperatures.